The invention relates generally to integrated circuit fabrication and, in particular, to methods for real-time monitoring of an attribute of a photomask.
Semiconductor manufacturers fabricate integrated circuits using photolithographic processes that rely upon a series of different photomasks to define different structural features characteristic of the circuit. The pattern engrained on each photomask is designed to conform to dimensional rules that ensure that the patterns transfer properly and that the integrated circuit functions properly. Once the layout is created as a pattern on the photomask, the photolithographic process utilizes an exposure tool to project the mask pattern onto a photoresist layer carried on the semiconductor wafer.
Among other process variables, a satisfactory product yield is contingent upon having essentially defectless masks and reticles. Photomask deployment may result in relatively high mean time to detect (MTTD) for the appearance of new mask-related defects or the occurrence of mask degeneration. Poor handling of a photomask may dislodge otherwise innocuous particulates and cause relocation to positions on the photomask that ultimately cause defects in the wafers fabricated using the photomask. In any event, degradation of the photomask from contamination, as well as an abnormal environmental factor or events occurring during handling, may cause yield loss in the integrated circuits fabricated using the photomask.
As an important step associated with the photolithography process, wafer fabricators periodically inspect every photomask after a fixed number of wafer passes to determine the condition of the photomask. Unfortunately, high-resolution mask inspection tools, such as CCD imaging systems, are typically only found in mask fabrication shops. Because of the absence of a high-resolution mask inspection tool, wafer fabricators may have only a limited ability to monitor the photomask condition. In any event, conventional approaches for photomask inspection do not provide real-time monitoring of contamination on the photomask, the environment of the photomask, or a physical attribute of the photomask.
Accordingly, there is a need for improved methods to provide monitoring of a measurable contamination, environmental, or physical attribute of a photomask.